Most small Cold Cathode Fluorescent Lamps (CCFLs) are used in battery powered systems. The system battery supplies a direct current (DC) to an input of a DC to AC inverter. A common technique for converting a relatively low DC input voltage to a higher AC output voltage is to chop up the DC input signal with power switches, filter out the harmonic signals produced by the chopping, and output a sine-wave-like AC signal. The voltage of the AC signal is stepped up with a transformer to a relatively high voltage since the running voltage could be 500 volts over a range of 0.5 to 6 milliamps. CCFLs are usually driven by AC signals having frequencies that range from 50 to 100 kilohertz.
The power switches may be bipolar junction transistors (BJT) or Field Effect Transistors (FET or MOSFET). Also, the transistors may be discrete or integrated into the same package as the control circuitry for the DC to AC converter. Since resistive components tend to dissipate power and reduce the overall efficiency of a circuit, a typical harmonic filter for a DC to AC converter employs inductive and capacitive components that are selected to minimize power loss. A second-order resonant filter formed with inductive and capacitive components is referred to as a “tank” circuit, since the tank stores energy at a particular frequency.
The average life of a CCFL depends on several aspects of its operating environment. For example, driving the CCFL at a higher power level than its rating reduces the useful life of the lamp. Also, driving the CCFL with an AC signal that has a high crest factor can cause premature failure of the lamp. The crest factor is the ratio of the peak current to the average current that flows through the CCFL.
On the other hand, it is known that driving a CCFL with a relatively high frequency square-shaped AC signal maximizes the useful life of the lamp. However, since the square shape of an AC signal may cause significant interference with another circuit disposed in the immediate vicinity of the circuitry driving the CCFL, the lamp is typically driven with an AC signal that has a less than optimal shape such as a sine-shaped AC signal.
Double-ended (full-bridge and push-pull) inverter topologies are popular in driving today's discharge lamps because they offer symmetrical voltage and current drive on both positive and negative cycles. The resulting lamp current is sinusoidal and has a low crest factor. These topologies are very suitable for applications with a wide DC input voltage range.
The cost of double-ended designs, however, remains a main concern for low power and regulated input applications. Full-bridge circuits require four power switches and complicated drive circuits. Push-pull inverters require two power switches whose voltage rating must be greater than twice input voltage, and use a snubber circuit to suppress the leakage inductance-related ringing, where a snubber circuit is connected around a power device for altering its switching trajectory, usually for reducing power loss in the power device.
Single-ended inverters are therefore considered for a low-power and cost-sensitive application. Traditional single-ended inverters do not offer the symmetrical voltage waveform to drive the lamp, even if the duty cycle is close to 50%. In addition, the traditional circuit requires an expensive high voltage and high current resonant capacitor on the primary side and high voltage MOSFET to sustain the resonant voltages. Therefore, the traditional single-ended inverters do not offer a significant cost advantage over the double-ended inverters in addition to the fact that their performance is not as good. There is a need for single-ended inverters to efficiently drive discharge lamps at low cost, particularly for applications with a narrow input voltage range.